TISSUES AND SIMPLE ORGANS. Ill 



It might have been stated above that high negative pressures 

 have not been observed in the tracheal system at a considerable 

 height (of a tree) ; neither have high positive pressures been ob- 

 served in the lower part of a tree-trunk. This evidently tends to 

 prove that capillarity is not the moving force. It is certain, how- 

 ever, that spaces containing rarefied air do occur in the vessels or 

 in the tracheal system. The question that arises is, What can en- 

 dosmosis, as the result of rarefying air, accomplish in the tracheal 

 system by way of elevating water? In regard to this question 

 many authors hold erroneous opinions. SCHWENDENEB, ZIMMER- 

 MANN, * and GODLEWSKI a have opposed these opinions. I will here 

 only add from the recent work of Schwendener 8 that the lifting 

 power due to suction, assuming the water-columns to be 10 mm. 

 long and the air-columns to be of equal length, may be from 

 13 to 15 m. However, the water-columns observed in trees after 

 the time of " bleeding " were much less than 10 mm. 



We are approaching the close of this subject, and shall now 

 state that which at present seems to be the authoritative final ex- 

 planation of the water-movement in plants : dead elements are 

 essentially the paths in which water moves, while the living cells 

 supply the propelling force for the transpiratory current of water. 



Experiments which show that water movements in a living 

 tree will also continue through dead segments (killed by steaming, 

 poisoning, etc.) do not prove that living cells are unnecessary to 

 the ascent of sap (Schwendener) ; such dead portions presumably 

 contain more than the usual quantity of water in the tracheal sys- 

 tem at the beginning of the experiment, later, very likely, Jamin- 

 chains. 



The following is additional evidence of the correctness of the 

 foregoing fundamental ideas : the contact and relation of commu- 

 nication between the dead tracheal system and the system of living 

 cells in the vascular bundles and plant-organs; the occlusion of 

 vessel-lumina by means of tyloses and callus in the case of injuries ; 

 and the reduction or almost entire absence of vessels in submerged 

 water-plants whose needs for a water- conducting system are very 



1 Ber. der Deutsch. Bot. Gesellsclmft I, 1883. 

 * Pringsbeim's Jahrbucber XV, 1884. 



3 Zur Kritik der neuesten Untersuchungen tiber das Saftsteigen, Sitzungsber. 

 der Berl. Akad., 1892. 



